1. Field of the Invention
Rocket thrusters are commonly used to control the flight attitude and velocity of both manned and unmanned spacecraft while in flight as well as for carrying out various maneuvers of the spacecraft.
This invention relates to techniques for fabricating the nozzles used in such rocket thrusters. More particularly, the present invention relates to new and improved fabrication techniques for forming rocket thruster nozzles from electrically conductive materials as well as for cutting cooling channels in the internal walls of the nozzles.
2. Background Art
Heretofore the nozzles for rocket thrusters have been fabricated with conventional mechanical cutting or grinding tools. Since these nozzles are typically subjected to extreme gas temperatures in the order of 6,000-degrees F., it is preferred to use heat resistant materials such as sintered carbides or refractory metals. However, since these materials have very high hardnesses, are brittle and otherwise difficult to form, it is difficult and expensive to manufacture these nozzles. One common practice used heretofore has been to employ various molding, casting or forging techniques for forming these heat resistant materials into suitable nozzle shapes. Nevertheless, with even the best forming techniques, these nozzles require considerable grinding and machining to properly shape their critical internal surfaces.
As explained, for example, in U.S. Pat. No. 4,639,568, another technique for forming precision nozzles employs a specially-shaped cold-forming punch to initially shape the frustoconical interior portion of a nozzle. The nozzle is then completed by utilizing a so-called "EDM" or electrical discharge machine which has a frustoconical electrode that is coaxially positioned in the previously-formed cavity and the EDM machine is operated to precisely form the wall surfaces without contacting the nozzle walls.
U.S. Pat. No. 4,069,978 discloses a similar technique for forming offset passages which are inclined in relation to the central bore of the nozzle. After the central bore is formed, an inclined pilot hole is drilled completely across the nozzle body and appropriately directed so as to intersect the central bore. An elongated EDM electrode is then positioned in the pilot hole and operated to enlarge and shape one portion of the pilot hole as needed for defining an offset passage in one side of the nozzle body extending between the central nozzle bore and the exterior of the nozzle body. Once this offset passage has been properly shaped, the electrode is withdrawn and the unwanted portion of the pilot hole in the other side of the nozzle body is permanently plugged with a metal plug.
In another fabrication technique which is described in U.S. Pat. Nos. 4,508,604 and 4,578,556, a work piece is mounted on a work table which is adapted to be moved along orthogonal X-Y axes by a pair of electric motors that are controlled by a numerical control unit which is programmed for moving the work piece along a predetermined path in the X-Y plane. Once the work piece has been mounted on the table, a longitudinally-movable EDM wire electrode is positioned in a previously-formed hole in the work piece. Thereafter, as the work piece is being transported along the X-Y plane, the forward edge of the vertically-moving electrode element will be progressively cutting away the adjacent vertical surfaces of the work piece. In this way, during the cutting operation, the hole in the work piece will be progressively enlarged and shaped in accordance with the programmed cutting pattern of the numerical control unit.
It will, of course, be appreciated by those skilled in the art that despite the aforementioned advances in the prior art, it has not been considered possible heretofore to efficiently employ these prior-art techniques for fabricating the nozzles for rocket thrusters from heat resistant materials such as the materials mentioned above. For example, the use of an EDM electrode that is specially shaped to form the throat portion of a thruster nozzle is not particularly efficient since the electrode tends to spall the adjacent surfaces of the nozzle throat. On the other hand, since a typical travelling-wire EDM machine such as shown in the aforementioned patent is limited to cutting only along the Z-axis and the work piece is movable only in the X-Y plane, apparently it has not been considered possible to use these EDM machines for cutting the diverging and converging surfaces of a thruster nozzle.
Moreover, in view of the extreme operating temperatures typically experienced with rocket propellants, it is preferred to provide a plurality of circumferentially-spaced channels in the internal walls of these nozzles through which a coolant or some of the rocket propellant will be passed for cooling the nozzles during their operation. It will be appreciated, of course, that with typical thruster nozzle materials, it is no simple task to precisely form a group of small cooling channels in the walls of these nozzles. More particularly, it should also be realized that the size and shape as well as strength of any cutting tool being used to cut the cooling channels will limit their size, shape and locations. It should also be noted that if a typical EDM machine with a longitudinally-movable electrode is used to cut the cooling channels, the geometry of the nozzles will ordinarily result in these channels being cut at varying depths into the wall of a nozzle. The resulting thickness variations of the remaining material in the nozzle wall will often result in uneven or inefficient cooling of the wall as well as produce serious thermal stresses in the nozzle when the thruster is operated. Accordingly, it is believed that it is not particularly effective to employ these prior-art fabricating techniques either for shaping the internal contour of a thruster nozzle or for forming a plurality of intricate cooling channels in its internal walls.